Current transformer



Oct. 18, 1960 J, 5, JENNINGS ETAL 2,957,151

cumm TRANSFORMER Filed Feb. 111 1957 INVENTORS JO EMMETT JENNINGS BYHUGH 0. R088 their ATTORNEY United States Patent CURRENT TRANSFORMER JoEmmett Jennings and Hugh C. Ross, San Jose, Calif.,

assignors to Jennings Radio Manufacturing Corporation, San Jose, Calif.,a corporation of California Filed Feb. 11, 1957 Ser. No. 639,537

2 Claims. (Cl. 336-174) Our invention relates to electricaltransformers; and particularly to a vacuum insulated currenttransformer. One of the objects of our invention is the provision of avacuum insulated transformer characterized by close spacing of theprimary inductor and secondary element.

Another object of the invention is the provision of a high potentialvacuum insulated current transformer of small size and light weight.

Still another object is the provision of a substantially fireproofvacuum insulated transformer.

A still further object is the provision of a vacuum insulatedtransformer in which the secondary element may be easily replaced byanother possessing a different current rating.

Another object is the provision of a vacuum insulated transformercapable of measuring with equal case either open or shielded typecircuits.

A further object is the provision of a vacuum insulated transformeruseful over a wide range of Voltages without increasing the size of theprimary inductor or secondary element.

.A still further object is the provision of a vacuum insulatedtransformer capable of handling test voltages approximately five timesas high as the rated working voltage of the primary inductor.

Another object is the provision of a vacuum insulated to the showingmade by the said description and the drawings, since we may adoptvariant forms of the inven tion within the scope of the appended claims.

Referring to the drawings:

, Fig- 1 is a vertical half-sectional view of one form 'of our vacuuminsulated transformer.

Fig. 2 is a vertical half-sectional view showing a modi- -fied form ofour vacuum insulated transformer.

. Both figures are drawn approximately one-half actual size.

In the measurement of large, low potential alternating currents it iscommon practice to employ a device known as a current transformer. Theprimary conductor of this device is inserted in series with a circuitconductor, and by measuring the proportional voltage and current inducedin the secondary of the transformer, it is possible to calculate thecurrent flowing through the circuit conductor. Measurement of currentsin high potential conductors however, presents difliculties due to theinherent limitations in conventional transformers, which require widespacing between the primary and secondary conductors, and a large amountof insulation therebetween to render them safe under high potentialoperation. As potentials are increased, the spacing and amount ofinsulation must be correspondingly increased. As the spacing isincreased, however, the ability of the transformer to perform itsintended function is reduced, thus destroying element of thetransformer.

the efficiency of the conventional current transformer and rendering itunsuitable for the measurement of high potential currents.

To obviate the disadvantages inherent in conventional currenttransformers, we have provided a current transformer or vacuum potheadcharacterized by close spacing of the primary inductor and secondaryelement to provide high efficiency; and effective vacuum insulationinterposed between the primary and secondary, rendering our transformersafe and useful for both low and high potential current measurement.

Broadly considered, our vacuum insulated current transformer comprises aconductor constituting the primary inductor of the transformer,supported within a vacuumized envelope. Terminals outside the envelopeare conductively connected to the primary inductor and serve to seriallyconnect the device into an alternating circuit. Inductively encirclingthe primary inductor and positioned outside the vacuumized envelope isan annular coil constituting the secondary element of the transformer.Alternating current flowing through the primary inductor thus induces aproportional alternating voltage and current in the secondary element.Connecting the secondary element with an appropriately calibratedvoltmeter or ammeter will thus given a direct reading of the potentialor current flowing in the conductor under test. The ability to safelyand efiiciently handle large potentials is augmented by enlarging oneend of the envelope and providing it with a series of spaced circularcorrugations or convolutions.

More specifically, the vacuum insulated current transformer of ourinvention comprises an evacuated envelope formed by a hollow, generallycylindrical dielectric shell 2, hermetically closed at opposite ends bymetallic end caps 3, hermetically united with the dielectric shell. Theshell illustrated is conveniently formed from high dielectric strengthglass hermetically united with the mutually reaching end cap flanges 4by conventional metal-toglass seals. Where the use of a ceramic shell isdesirable, the end caps may conveniently be brazed on the metallizedends of the ceramic shell.

Mounted within the envelope and conductively extending between the metalend caps is a conductor 6, constituting the primary inductor orconductor of the transformer. The primary inductor is convenientlycylindrical and brazed at each opposite end within a hollow externallyprojecting boss 7 on each end cap. Axial alignment of the primaryinductor with the enclosing shell is preferred, with the primaryinductor extending the full length of the envelope. The end cap bosses 7serve as terminals for connecting the transformer into an alternatingcircuit.

Encircling the primary inductor in spaced inductive relation is anannular coil 8 constituting the secondary The coil is wound on a flangedcylindrical hub 9 proportioned to slidably engage the outer peripheralsurface of the shell. A bead '12 integrally formed on the shell to oneside of its midpoint functions as a stop to limit downward movement ofthe coil when the device of Fig. 1 is mounted in a vertical position asshown. It will be understood that the device may be positionedhorizontally if desired, in which case the bead 12 serves to positionthe coil midway between the ends of the envelope.

Slidability of the secondary coil on the outer peripheral surface of theenvelope permits its ready detachability and replacement by differentlyrated coils, thus widely extending the range of applicability of thedevice. In some installations the coil may also function as a supportfor the envelope, leaving the envelope ends free for connection into thecircuit. In this case, the coil is independently supported on a suitablebracket or panel as in Fig. 2. Electrically connecting the secondarycoil to an appropriately calibrated ammeter 13, results in a dlrectindication of the current load in the circuit being tested or metered.

In the modified form of our vacuum insulated transformer or potheadshown in Fig. 2, the bead 12 has been omitted and the transformer issupported in a panel 14 equipped with a flanged collar 16 secured on thepanel by screws 17, and slidable on the cylindrical periphery of theshell 2. At one end the glass shell 2 is enlarged diametrically andprovided with a plurality of circular, axially spaced corrugations orconvolutions 18 integrally formed in the shell. This constructioneliminates corona loss, which is the prolific source of radio andtelevision interference, and increases the external resistance over thesurface of the envelope.

Conventional transformers or potheads commonly use dielectric compoundssuch as transformer oil to reduce corona loss. These oil-filled deviceshave proven unsatisfactory due to their tendency to leak the oil, whichacts as a dirt catcher, eventually resulting in a flashover of potentialon the surface of the pothead. Also, conventional potheads are generallybrown-glazed and opaque, and cannot be inspected internally. The vacuurninsulated transformer or pothead of our invention, when formed of clearglass, permits observation during abnormal operating conditions withcomplete safety to the observer, whereas oil-filled potheads arepotential fire hazards, tending to explode outwardly when they are overinternally.

Manufacture of our vacuum insulated transformer to meet thespecifications of the National Electrical Code requires that eachinstrument be tested at a voltage approximately five times the workingvoltage of the primary inductor. For example, our vacuum insulatedtransformer designed to operate in a kv. circuit, must be tested at avoltage of approximately 75 kv. rms. Translated into peak values, thiswould mean that the peak voltage would be about 106,000 volts.

In order to protect the dielectric shell against destructiveelectrostatically imposed strain at these high peak test voltages, anelectrostatic shield is provided. The shield may be a true Faradayshield having a plurality of slits in its periphery, or it may beformed, as illustrated, from a closed cylindrical portion of coppertubing 19. The cylindrical shield is hermetically sealed Within theenvelope, and is proportioned to fit close against the inner surface ofthe shell between the shell and the primary inductor, and in axialalignment with both. The length of the shield is proportioned so thatthe shield will project on both sides of the coil. The shield thus willlie wholly within the magnetic field projected from the centrallydisposed primary inductor. Since the field density is constantlyvarying, an intermittent electrostatic charge will be induced on theshield. The electrostatic charge is channeled to ground as alternatingcurrent through a hermetically sealed terminal post 21. The inner end ofthe post 21 is conductively connected with the shield, and functions asa means for fixing the shield in position within the envelope. In Fig. 1the inner end of the post is brazed directly to the shield, and in Fig.2, the conductive connection is conveniently accomplished by a heavylead22. During the testing operation the outer end of the post isconveniently connected to ground by conductor 23. As a safetyprecaution, the shield may also be grounded during normal operation.

We claim:

1. A vacuum insulated current transformer comprising an elongatedgenerally cylindrical unitary dielectric envelope, metallic end caps foreach end of the cylindrical envelope having a cup-like shape with abottom and generally cylindrical wall hermetically sealed to the ends ofsaid envelope, said bottom of the end caps being formed with a recesscentrally disposed therein and protruding outward from the bottom formounting a primary inductor, said envelope being evacuated andhermetically sealed, a cylindrical conductor forming the primaryinductor of the transformer and extending coaxiall'y within saidenvelope and supported at each end in said recess formed in the end capsfor receiving and supporting the same, an annular coil constituting asecondary element of the current transformer and slidably received oversaid evacuated envelope in spaced coaxial inductive relation to saidcylindrical conductor, said cylindrical wall of the envelope havingformed therein an outwardly protruding portion intermediate the ends ata position to form a stop means for positioning said coil at a middleposition along said elongated envelope, and an electrostatic shieldingmeans of generally cylindrical shape mounted concentrically about saidprimary inductor and in abutting relation to the surrounding inside ofsaid envelope and extending axially beyond each end of the secondarycoil member and having ground conductor means connected thereto andextending through the wall of said envelope and externally thereof. 2. Avacuum insulated current transformer according to claim 1 wherein oneend of said evacuated envelope is of uniform diameter which slidablyreceives in adjustable position therealong said annular coil and thebalance of said evacuated envelope is formed with a plurality of spacedcorrugations therein, the corrugation nearest the center of the envelopebeing positioned to form stop means for positioning said coil formingthe secondary element at a middle position along said elongatedevacuated envelope.

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